1. Field of the Invention
The present invention relates generally to magnetic data storage media having a protective carbon overcoat. In particular, the present invention relates to an improved thin carbon overcoat for magnetic data storage disks and a method for the fabrication of an improved carbon overcoat.
2. Description of Related Art
Thin film magnetic recording media, such as hard disks, are utilized to store large amounts of data and are typically composed of multiple layers, including a rigid substrate, an underlayer and one or more magnetic layers. The magnetic recording layer includes a polycrystalline magnetizable material that is locally magnetized by a read/write head that traverses the surface of the recording medium during operation of the disk drive.
When a disk drive is in operation, the disk rotates at a high rate of speed and the read/write head flies over the surface of the hard disk as it rotates. The read/write head rests against the surface of the hard disk when the disk is not rotating. When the disk is activated and begins to rotate, the read/write head rises above the disk surface on a bearing of air and can be moved in a controlled fashion to read and write data on the disk. When the disk drive is turned off and the disk stops rotating, the head again comes into sliding contact with the disk surface. These surface contact events can damage the surface of the hard disk, including the magnetic recording layer. Accordingly, a carbon overcoat is typically deposited over the magnetic recording layer to protect the recording layer during start and stop operations. The carbon overcoat also protects the magnetic recording layer from corrosion. A thin layer of lubricant is applied over the protective carbon overcoat to improve the interaction between the read/write head and the disk surface and to improve the tribological performance of the hard disk drive. The liquid lubricant must adhere to the carbon overcoat such that the rotational velocity of the disk does not cause a significant amount of the lubricant to be lost from the disk surface.
The carbon overcoat layers are typically formed by sputtering. For example, U.S. Pat. No. 5,607,783 by Onodera discloses a magnetic data storage hard disk including a carbon protective film over the magnetic layers. The hydrogen content of the carbon film near the magnetic layer is lower than the hydrogen content near the lubrication layer. It is disclosed that the portion of the carbon layer with a low hydrogen content shows high hardness and that the upper carbon layer with a high hydrogen content shows more flexibility. The carbon layer has a thickness of about 180 angstroms and is formed by sputtering in an atmosphere containing methane gas (CH4).
U.S. Pat. No. 5,785,825 by Hwang et al. also discloses a magnetic hard disk including a carbon overcoat layer. The carbon overcoat is a dual phase carbon overcoat including an amorphous carbon film sputtered on a magnetic layer and a doped amorphous carbon film sputtered on the amorphous carbon film. The doped amorphous carbon film can include hydrogen or nitrogen. The total thickness of the dual phase carbon overcoat is about 200 angstroms.
U.S. Pat. No. 5,855,746 by Prabhakara et al. also discloses a magnetic hard disk including a carbon overcoat layer. The carbon overcoat includes a first buffer layer adjacent to the magnetic layer that does not include nitrogen and a nitrogenated carbon layer over the buffer layer. The buffer layer is adapted to protect the magnetic layer from damage due to the nitrogen. The carbon overcoat has a total thickness of about 110 angstroms and the individual layers of the carbon overcoat are each formed by sputtering.
U.S. Pat. No. 5,858,182 by Horng et al. discloses a magnetic hard disk including a wear resistant carbon overcoat layer and a lubricating carbon layer. The wear resistant carbon layer has a thickness of from 40 to 200 angstroms and is formed from nitrogenated wear resistant carbon materials and hydrogenated wear resistant carbon materials. A lubricating carbon layer is disposed over the wear resistant layer and has a thickness of 20 to 50 angstroms and contains 30 to 40 atomic percent hydrogen.
U.S. Pat. No. 5,945,219 by Yamada et al. discloses a magnetic recording medium of the metal film type including a carbon overcoat that has rust proofing and lubricating properties. The overcoat is formed by chemical vapor deposition (CVD) wherein a lubricant and a rust proofing agent are introduced during the CVD process. The overcoat has a total thickness of about 100 angstroms.
U.S. Pat. No. 6,086,730 by Liu et al. discloses a magnetic hard disk including a carbon overcoat. The carbon overcoat is formed by pulse sputtering and includes an amorphous hydrogenated carbon thin film with a high sp3 bond content, which is indicative of diamond bonding and high hardness.
U.S. Pat. No. 6,312,798 by Ma et al. discloses a magnetic hard disk including a carbon overcoat layer. The carbon overcoat layer includes a nitrogen doped carbon hydrogen film that is formed by ion beam deposition. It is disclosed that the nitrogen lowers the resistivity of the film, thereby eliminating charge buildup and reducing glide noise. The film has a total thickness of from about 20 to 80 angstroms.
U.S. Pat. No. 6,358,636 by Yang et al. discloses a magnetic hard disk that includes protective overlayers. The overlayers include a transition metal-silicon or transition metal-germanium interlayer and a carbon overcoat. It is disclosed that the carbon overcoat has a thickness of at least about 30 angstroms can be formed by sputter deposition, as well as ion beam deposition or CVD.
There is a continuing demand to increase the recording density (i.e., areal density) of magnetic hard disks to enhance data storage capacity. One technique to increase the areal density includes decreasing the spacing between the read/write head and the magnetic layer. This spacing can be reduced by reducing the thickness of the carbon overcoat. However, the carbon overcoat must remain sufficiently robust to provide effective protection against wear due to head-disk surface contact events and effective protection against corrosion.
The carbon overcoat includes an interface between the magnetic layer and the carbon overcoat, as well as an interface between the carbon overcoat and the lubricant. The mechanical and chemical properties of the carbon overcoat and the surface reactivity of the carbon overcoat affect the nature of the carbon-lubricant bonding, which subsequently affects the interaction between the lubricant and the read/write head.
Technologies such as ion beam deposition, cathodic arc deposition, and plasma-enhanced chemical vapor deposition (PECVD) are capable of producing hard carbon overcoats with a high fraction of sp3 bond content. For example, ion beam carbon (IBC) deposition technology can be used to produce protective carbon overcoats having improved properties as compared to sputtered carbon. Ion beam carbon formed using a high impact energy ion beam provides a higher fraction of sp3 content in the carbon overcoat that leads to high hardness, high density, high elastic modulus, low friction and improved chemical inertness. Ion beam carbon also has a higher resistance to tribochemical wear than sputtered carbon.
However, hard carbon overcoats lead to severe glide head-disk interactions (i.e., glide noise) and head degradation. Further, ion beam carbon often has a high hydrogen content. A high hydrogen content increases the electrical resistivity of the overcoat which can lead to charge buildup on the surface, also resulting in head damage. High hydrogen content in the overcoat can also decrease the dangling bond density in the carbon layer, resulting in lower carbon lubricant bonding and pick-up of lubricant by the read/write head. The physical and chemical properties of both the carbon overcoat and the lubricant affect the glide noise and head degradation. The mechanical/chemical nature of the carbon overcoat also affects the degradation of the lubrication layer, which is important for improving wear durability at the head-disk interface.